Relatively thin filter beds, called precoat layers or precoats, are widely utilized to improve the efficiency of mechanical filtration and adsorption. The precoat is deposited on the surface of a porous septurn, such as a filter screen, an annular filter cartridge, a tubular filter cartridge, a pleated filter cartridge, a deep or shallow bed, or the like. The precoat enhances removal of contaminant particles from a liquid stream, protects the septum from plugging, and retains the particles on or upstream of the surface of the septum so that the particles can be more easily dislodged by backwashing. These advantages are offset during operation to some extent by an increase in overall pressure differential across the precoat and the septum.
Some water treatment applications require the removal of both dissolved and suspended or colloidal materials. Steam generation of electrical power, in both fossil fuel and nuclear power plants, exemplifies such water treatment applications. In both applications, the recycle stream for the steam turbines is usually passed through a filter bed.
A significant breakthrough in the purification of liquid streams is described in U.S. Pat. Nos. 3,250,702 and 3,250,703, both of which are assigned to the assignee of this application. The invention described in these patents is based on the discovery that, when finely divided anion and cation exchange resin particles are mixed in aqueous suspension, a volume increase is noted. This volume increase is the result of an agglomeration or "clumping" between the anion and cation exchange resin particles. Such resin particles, when used to form a filter bed, produce significantly reduced pressure drops across the bed, together with longer run lengths and improved efficiency of dissolved and undissolved solids removal.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises a mixture of oppositely charged particles of filter aid material of oppositely charged particles of filter aid material is described in U.S. Pat. No. 4,177,142, which patent is assigned to the assignee of this application. The filter aid materials are disclosed as including diatomaceous earth, cellulose fibers, charcoal, expanded pearlite, asbestos filters, ion exchange resins, and inorganic ion exchangers. The filter bed comprises a mixture of oppositely charged particles of filter aid material. The filter aid particles normally have a surface charge in aqueous suspension. A portion of the particles is treated with a chemical compound to produce a surface charge which is opposite to the normal surface charge. A mixture of oppositely charged particles (normal and reversed) is therefore produced, and the "clumping" phenomenon is achieved.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises treated filter aid material mixed with finely divided ion exchange resin particles in the size range of 60 to 400 mesh is described in U.S. Pat. No. 4,190,532 which patent is assigned to the assignee of this application. The mixture of treated filter aid material and ion exchange resin particles produces a dumping phenomenon. The filter aid materials include cellulose fibers, diatomaceous earth, charcoal, expanded pearlite, asbestos fibers and polyacrylonitrile fibers. The ion exchange resin particles include cation exchange resins, anion exchange resins or a mixture of both. The filter aid materials are treated with a chemical compound to produce the required surface charge.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises a mixture of a treated fibrous filter aid material and an active particulate material is described in U.S. Pat. No. 4,238,334, which patent is assigned to the assignee of this application. The "fibrous filter aid materials" include cellulose fibers, polyacrylonitrile fibers, Teflon fibers, nylon fibers, rayon fibers, polypropylene fibers and polyvinyl chloride fibers.
In U.S. Pat. No. 4,747,955, assigned to the assignee of this invention, there is described a method for removing impurities wherein polyester fibers are used as a filter aid material in precoat filter formulations. The polyester fibers are treated with a hot caustic solution to covert the normally hydrophobic surface of the fibers to a hydrophilic surface.
In U.S. Pat. No. 4,313,832 there is described a method for purifying aqueous solutions by the use of ion exchange fibers. The ion exchange fibers have a thickness in the range of from about 2 to about 200 .mu.m and a length of more than twice the thickness. In order to intertwine the ion exchange fibers into a compact mass, they are mixed with water and stirred thoroughly. The compact mass of fibers is applied as a slurry to a filter support to form a precoat layer thereon. The ion exchange fibers may be cation exchange fibers, anion exchange fibers or a mixture thereof. An alternative embodiment is disclosed wherein the intertwined ion exchange fibers have finely divided ion exchange resin particles of a diameter from about 2 to about 250 .mu.m entrained between the ion exchange fibers.
All known precoats increase the total pressure differential observed across the precoat and the septum during filter operation. Generally, precoats which are relatively more compressible exhibit higher total pressure differentials and greater filtration efficiencies. However, higher pressure differentials consume pumping energy and limit the flow rate of fluid which can be passed through the filter. A need exists for a precoat that provides relatively great filtration efficiency while simultaneously exhibiting a relatively low operating pressure differential.